20 Fossils That Scientists Completely Misinterpreted—and What It Took to Finally Set the Record Straight

In 1868, Edward Drinker Cope described this Upper Cretaceous plesiosaur based on fossils discovered in Kansas. Unfortunately, he depicted the dinosaur upside down, placing the skull at the opposite end. He thus made a widely publicized mistake by placing the skull at the opposite end. The long row of vertebrae belonged to the neck, not the tail.

In 2015, Tetrapodophis made headlines: this tiny Brazilian fossil with four limbs appeared to provide evidence of an early stage in the evolution of snakes. This would have been a major discovery, as the origins of snakes are still a subject of debate today. However, a subsequent study of the fossil’s anatomy revealed that this Lower Cretaceous animal actually belonged to the lizard family.

Hallucigenia, a Cambrian lobopod, was discovered in the Burgess Shale, a fossil-rich deposit dating back approximately 508 million years located in British Columbia. Initially, researchers had positioned it upside down, mistaking its spines for legs and its soft appendages for structures along its back.

Archaeoraptor was presented in 1999 as the “missing link” between dinosaurs and birds, based on a fossil discovered in China. This theory collapsed when researchers realized that the specimen had been pieced together from parts of different animals. Instead of being a major discovery for evolutionary science, it has become a frequently cited example used to warn against fossil trafficking.

It was Othniel Charles Marsh who named this animal “Brontosaurus” in 1879. More than twenty years later, Elmer Riggs argued that this long-necked herbivore was too similar to Apatosaurus to warrant its own genus—a classification that remained in place until 2015. Today, these two Upper Jurassic animals each belong to their own genus.

Helicoprion was a shark-like fish that lived about 280 million years ago, best known for its spiral-shaped lower jaw. Some of the earliest reconstructions placed this unique jaw where its dorsal fin would have been, until a 2013 confirmation put this row of teeth back in its proper place.

This Upper Cretaceous herbivore was first discovered in 1948 in what is now Mongolia. The only bones found were those of the forelimbs, enormous claws, and ribs, leading scientists to believe that the animal might have been a sea turtle or a raptor.

Basilosaurus, or “king lizard,” which lived at the end of the Eocene, was given this name in 1834. Its long vertebrae resembled those of a reptile so closely that early researchers believed they were dealing with a giant marine reptile. Subsequent anatomical studies showed that Basilosaurus was in fact one of the earliest whales.

Coelophysis, a small Triassic dinosaur native to the southwestern United States, has been linked to cannibalism following the discovery of small bones near adult skeletons. For a time, this made it one of the best-known examples of dinosaur behavior. Subsequent analyses showed that the remains thought to be those of juvenile Coelophysis were more likely those of small crocodylomorph reptiles, the distant ancestors of crocodiles and alligators.

Tanystropheus is best known for its extremely long neck. When it was first described in 1886, scientists mistook it for a pterosaur, a flying reptile from the Mesozoic Era. CT scans later made it possible to reconstruct its skull and confirmed the hypothesis that this large species was an aquatic ambush predator, with features adapted for catching fish and squid in coastal waters.

The Thylacoleo, often called the “marsupial lion,” was a apex predator in prehistoric Australia. This animal was characterized by an unusual skull, powerful forelimbs, and blade-shaped premolars. Initially thought to be a slow-moving ambush predator, later studies have shown that it was a powerful carnivore of the Pliocene and Pleistocene epochs.

This camel-like animal, dating from the Pliocene and Pleistocene epochs, was first discovered in 1834. At the time, researchers believed it to be a mastodon, the distant ancestor of elephants. Subsequent DNA and protein analyses allowed it to be classified among the extinct South American ungulate mammals, thus linking it to the genus of rhinoceroses and horses.

This Pleistocene primate was first described in 1935 following the discovery of its large teeth in Chinese herbal medicine shops. Initially thought to be part of the human lineage, these animals are now believed to be closely related to orangutans.

Our human-like ancestors lived from the middle to the end of the Pleistocene and became extinct about 40,000 years ago. Before the discovery of fossils of the ancient Cro-Magnons, it was believed that Neanderthals were part of human evolution. Of course, we now know that they were simply another human group that did not survive.

Dickinsonia lived during the Ediacaran Period, long before the seas were populated by animals as we know them today. Because it resembled a flat, ribbed oval, researchers spent decades debating whether it was an animal, a fungus, a lichen, or something entirely different. Chemical traces preserved in some fossils later supported the hypothesis that Dickinsonia was an animal that lived on the seafloor.

Conodonts were a group of marine vertebrates that existed from the Cambrian through the Jurassic periods. Since their teeth were the first parts to be discovered, this animal was initially mistaken for a worm, an alga, or a mollusk. It was later discovered that these teeth actually belonged to a primitive, eel-like vertebrate.

Pikaia was a Middle Cambrian fossil that was originally believed to be a species of marine worm. Subsequent studies revealed characteristics specific to chordates, including a notochord-like structure running through its body. This made this small animal much more interesting, classifying it as a chordate—a close relative of the ancestors of vertebrates.

Anchisaurus fossils were discovered in Connecticut in the early 19th century, long before dinosaurs were recognized as a distinct group. Some of these early fossils were even mistaken for human remains. Subsequent studies classified this Lower Jurassic animal as one of the earliest sauropodomorph dinosaurs, related to the lineage that eventually gave rise to giant, long-necked herbivores such as Brachiosaurus and Apatosaurus.

Archaeopteryx was first discovered in Germany in the 1860s, shortly after the publication of Darwin’s On the Origin of Species. This Upper Jurassic animal was initially thought to be a warm-blooded reptile, or even an animal more closely related to modern birds. Subsequent research has shown that it is the oldest known species of bird, related to other flying dinosaurs.

Megalonyx was a Pliocene-era animal that lived between five million and 13,000 years ago. Initially thought to be a fearsome predator because of its large size and imposing claws, it was later discovered that this gentle giant was actually a giant ground sloth.